As the focus this week turned to Barack Obama's choice of running mate, his other crucial partner continues to show that the Democratic nominee isn't the only Obama breaking down barriers. Michelle Obama is well on her way to becoming the first African-American first lady. But in a country that has a long way to go in terms of racial and gender justice, her reception has been far from warm. In the wake of the rank misogyny directed at Hillary Clinton throughout her campaign, it's time for American feminists to defend Michelle Obama with the same passion and vigour: the attacks are worse and the consequences more dire.

Media coverage of Obama has packed a nasty racism-sexism combo that is quickly becoming a national disgrace. First a Fox News segment captioned a picture of Michelle as "Obama's Baby Mama", a disparaging term for unmarried mothers. Rightwing commentators have called her an "angry black woman" and "Obama's bitter half", while the National Review ran a cover story dubbing her "Mrs Grievance". Then came the notorious, if satirical, New Yorker cover, where she was featured as having a large Afro and toting a machine gun.

Like it or not, it matters what the American public thinks of Michelle Obama. A recent poll showed 61% of Americans say the potential first lady plays a role in how they vote.

While there has been positive media - such as Michelle Obama's appearance on TV and on magazine covers - the underlying messages put out by the press reek of racism, sexism, and of a country unable, or unwilling, to see a black woman in a position of power. As one Washington Post commentator, Sophia A Nelson, wrote, for women who face this daily, it is far from shocking. "We're endlessly familiar with the problem she is confronting - being looked at, as black women, through a different lens from our white counterparts, who are portrayed as kinder, gentler souls who somehow deserve to be loved and valued more than we do."

Thankfully, this assault isn't going entirely unchecked. Blogs such as Michelle Obama Watch are holding news outlets accountable for inaccurate, sexist and racist characterisations. But that's not enough. It's time for feminists in the mainstream movement, many of whom are still stinging over Clinton's loss, to not just pull out all the stops for Barack, but to also use their considerable political power, money and influence to defend Michelle.

When Clinton was still campaigning I received daily emails from various women's organisations, furious about the way the media was treating their candidate. These days, however, my inbox is fairly empty. And the press releases and action alerts I do get about Michelle Obama seem to lack the outrage and fervour of past Clinton-focused statements.

That's not to say women's organisations aren't up to the task. The National Organisation for Women, for instance, has shifted the focus of their Media Hall of Shame to attacks on Obama. But feminists should be up in arms. Because as shameful as the treatment they're getting is, our silence is even more so.

Michelle Obama recently told a room full of women that "[we] need an advocate in the White House now more than ever before". And after eight years of the Bush administration, truer words were never spoken. But Michelle Obama also needs an advocate in us.

We have an opportunity to show the world that racism and sexism are not acceptable, that they have no place in public discourse, and that those with power will not just speak out against injustice - but will scream, full volume.

· Jessica Valenti is the founder of Feministing website and author of Full Frontal Feminism feministing.com